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Full text of "Text Book Of Mechanical Engineering"

Solid Friction.

555

Laws of Friction.—* Solid' friction (or the friction be-
tween solid surfaces) is here meant, in contradistinction to fluid
friction. There are three laws, as follows :

The tractive force required to overcome friction :—

(1)  Depends directly on the pressure between the surfaces in contact.

(2)  Is independent of the extent of the pair of surfaces in contact, but (20)

increases in proportion to the number of pairs of surfaces.

(3)  Is independent (at low velocities) of the relative velocity of the surfaces.

Further, the force depends on the co-efficient of friction (JJL)
for the particular materials, thus,

Tractive force Fn = p. P    where P = total pressure.
COEFFICIENTS (p) OF FRICTION AT VERY LOW SPEEDS.    (MORIN.)

MATERIALS.
	Method of Lubrication.
						
	Dry.
	Water.
	Olive oil.
	Lard.
	Tallow
	Dry soap.
	Polished and greasy.

* Wood on Wood. . .
 Metal on Metal... Wood on Metal... Hemp on Wood... ' Leather on Iron. . Leather on Wood Stone on Stone... Stone on W.I. ... Wood on Stone...
	"5 •18 •6 '63 '•54 '47 7* •45 •6
	•68
 •65 •By
	•12 "I
	•21 *I '12
	•19 •II
 *I2
	•36
	'35 "IS *I

			
	
	
	
	

		
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	•28

		
	
	
	
	
	
		
	
	
	
	
	

		
	
	
	
	
	

		
	
	
	
	
	

As yu is the trigonometrical tangent of the friction angle </>
(seep. 560), the latter may be found as in Fig. 570, by dividing
a base-line into tenths and setting up p. on a perpendicular
from the mark i, to the same scale. Thus, for dry metal,
^ =* 10°, when /* = *i8. (See Appendix //., p. 868.)